This invention relates to an indwelling stent used to ameliorate a stenosed part ocurring in a blood vessel, bile duct, trachea, esophagus, ureter, and other organs in the body, and the method for manufacturing the stent.
Many types of stents are hitherto proposed to maintain the openness of a vessel or a passageway.
Those stents can be divided into two types: self-expandable stents and balloon-expandable stents.
A balloon-expandable stent does not have a capability of expanding by itself. This type of stent is put in an intended stenosed part of the body by inserting a stent into the stenosed part and inflating the balloon in the stent to expand the stent (plastic deibrmation) dilating the stenosed part. Thus, this type of stent needs the expanding operation.
A self-expandable stent, on the other hand, has capability of expanding by itseff from a compressed state. This type of stent is put in an intended stenosed part of the body by inserting a stent in a compressed state into the stenosed part and removing the force maintaining the stent in the compressed state, fbr example, by holding a stent in a compressed state in a tube of a smaller outside diameter than the inner diameter of the intended stenosed part, advancing the front end portion of the tube near the stenosed part, and pushing the stent into the stenosed part out of the tube. The stent, released from the confinement of the tube, expands to resume the original shape, and dilates the stenosed part and is securely fixed in the part. This type of stent thus does not need the expanding operation, and the insertion operation of a stent is made easier.
Many type of self-expandable stents are hitherto proposed. Specifically, a super elastic metal stent is shown in J.P.B 5-43392 gazette. The catheter shown in J.P.B 5-43392 has a sheath which covers the outside she of the catheter on which it is placed, and has a stent (device fbr dilating a stenosed part) held between the front end portion of the catheter and that of the sheath. The stent used is such that the deformation both before and after it is put in the body is within the super elastic range. The shapes of the stent shown as usable are a coil (spiral), circular cylinder, roll, stepped pipe, high-order coil, flat-spring coil, cage, or mesh.
Thus being made of a super elastic metal which exhibits a super elastic property both before and after being put in the body, the stent shown in J.P.B 5-43392 is easy to insert into a stenosed part, and can continue to dilate the stenosed part for a long period because the stent steadily exhibits the super elastic property.
However, when the stent in the shape descn bed above expands, the ends of the stent move in the stenosed part. For example, a stent in the shape of a coil becomes shorter in the direction of the axis when expanded, and hence the ends of the stent moves toward each other. A stent in the shape of a roll on the other hand, does not change lengthwise in the direction of the axis, but the distance between the corners of each end of the stent changes as the stent unwinds. This move of the stent in the stenosed part can damage the inside surface of a body wall, and the damage can cause a new stenosis.
A stent in the shape of a simple circular cylinder has no problems as described above, but it is not easy to compress to a smaller outside diameter, and hence difficult to insert into a narrow stenosed part A stent in the shape of a stepped pipe is also difficult to compress to a smaller diameter, and further can hinder the blood flow and cause a new stenosis because of the parts projecting inward.
A stent in the shape of a cage or mesh is not easy to make of a super elastic metal. Moreover, the appropriate control of the resilience of the stent is very difficult, and this makes it difficult to determine the shape (dimensions) of the stent.
The first object of this invention is to provide an indwelling stent which is made of a super elastic metal exhibiting a super elastic property both in and out the body, easy to put in a stenosed part, does not hurt the body tissue when it expands in the stenosed part, and can reliably dilate the stenosed part and maintain the dilated state, and a method fbr manufacturing the stent.
Another self-expandable stent is shown in J.P.B 4-32662 gazette. This stent 30 is made of zigzags consisting of a plurality of straight segments and forming a closed loop (cylinder) as shown in FIG. 21. This stent has a problem that the entire stent becomes too rigid when the straight segments are made long in order to use it for a long stenosed part. Further, when two or more stents are put in a stenosed part as shown in FIG. 22, the part can rupture because of a too-large dilating power, the operation fbr putting in the stents is difficult, and the stents can move out of place.
The stent 40 shown in J.U.A 4-25755 is made by connecting two or more stents end to end by means of wires 41 as shown in FIG. 23 so that it can be put in a long stenosed part. Since individual stents constituting the stent 40 are independent, the stent 40 does not have a flexibility to bend along the sharply curved stenosed part.
The stent 50 shown in J.P.A 1-14506 is formed by a helically wound zigzag wire as shown in FIG. 24. This stent 50 can maintain the openness of even a bending stenosed part along the bend. However, since this stent 50 is made of a wire of a uniform rigidity, the dilating and maintaining force against the inside surface of a body wall is the same.
The degree of narrowing of a stenosed part 61 of a hollow organ or body cavity, specifially a blood vessel 61, is generally higher at the middle portion and becomes gradually lower toward the ends as shown in FIG. 7. Moreover, blood vessels 60 are straight only in a small part of them and bend in greater or lesser degree in almost all part of them.
If a stent with a uniform rigidity is put in a stenosed part, the ends of the stent can stimulate or hurt the endothelial cells to induce a hyperplasia and cause a stenosis again, especially when the rigidity is made high to dilate the stenosed part. If the rigidity is made low 80 as to give the stent an adequate flexibility, on the other hand, the stent may not reliably dilate the stenosed part.
The second object of this invention is to provide an indwelling stent whose middle portion has an adequate rigidity to dilate the intended stenosed part and maintain the openness and whose ends are flexible enough to bend along the bend of the body part without stimnulating the endothelial cells and causing a stenosis and which stably indwells the stenosed part without shifting out of place can be easily put in a stenosed part.
As described above, there are two type of stents: balloon-expandable stent and self-expandable stent.
A boon-expandable stent does not have a capability of expanding by itself, and is put in a stenosed part, fbr example, by inserting a stent into the intended stenosed part, positioning a balloon inside the stent, and inflating the balloon to expand the stent (plastic deformation) by the expansive of the balloon to secure the stent in close contact with the inside surface of the stenosed part.
Though this type of stent requires the operation for expanding the stent as described above, this is not so serious a problem, because the operation for expanding the stent can be accomplished comparatively easily by holding the stent around the deflated balloon. However, in this type of stent does not have an expanding resilience by itself, the inside diameter becomes smaller over time because of the pressure of the body part such as a blood vessel, causing a constriction again.
A self-expandable stent, on the other hand, has a capability of expanding by itself. This type of stent is put in the intended stenosed part of the body by inserting a stent in a compressed state into the stenosed part and removing the force maintaining the stent in the compressed state, fbr example, by holding a stent in a compressed state in a tube of a smaller outside diameter than the inner diameter of the intended stenosed part, advancing the front end portion of the tube near the stenosed part, and pushing the stent into the stenosed part out of the tube. The pushed out stent, released from the confinement of the tube, expands to resume the original shape. The stent thus dilates the stenosed part and is securely fixed in the part.
Thus having a capability of expanding by itself, this type of stent does not require the expanding operation, and has no problem that the diameter becomes gradually smaller over time because of the pressure of the body part, causing the recurrence of the stenosis.
This type of stent, on the other hand, is difficult to put in the intended stenosed part because of its expanding resilience. Accurate positioning of the stent is very difficult especially in thin, sharply bending blood vessels such as coronary arteries in which a flexible stent-inserting device (sheath and catheter) must be used.
A conventional stent-inserting device fbr a self-expandable stent is shown in FIG. 25 (J.P.A 62-501271).
This device comprises a probe 70, a catheter the front end portion of which is made in a double-wall structure by folding the wall inside out at the front end, the end 71a of the inside wall being attached to the probe 70, and a stent 72 held between the probe 70 and the catheter 71. This device is so constructed that the stent 72 is pushed out by the catheter 71 into the stenosed part when the probe 70 is moved frontward in the direction of the axis. This structure, however, is difficult to use in thin, sharply bending coronary arteries, because the portion in which the stent is held becomes rigid. Further, no device is made to accurately position the stent in a stenosed part in thin, sharply bending coronary arteries. Furthermore, since the very thin catheter must be folded inside out, this stent-inserting device is difficult to manufacture.
As described above, until today, there is no stentinserting device (assembly) which can put a self-expandable stent accurately in a stenosed part in thin, sharply bending hollow organs such as coronary arteries.
Therefore, the third object of this invention is to provide a stenosis-treating device which can put a selfexpandable stent easily and accurately in a stenosed part in thin, sharply bending hollow organs such as coronary arteries.